In wheeled vehicle axle suspension systems, it is often desirable to have a lifting feature that lifts the wheels of the system from road engagement when their load carrying capabilities are not needed. The liftable axles are lifted when operating the wheeled vehicle in a lightly loaded or empty condition and lowered when the vehicle is loaded sufficiently to require an extra axle for safety or to conform to highway weight laws.
In suspension systems for trailing axles and pusher axles, it is known that if a steerable axle is installed with the proper pitch or caster angle, the drag of the wheels will cause the axle to steer automatically in response to steering of the vehicle. This is referred to as a self-steering suspension system. It is also known in the art to provide a parallelogram arrangement of control arms or torque rods connected between a frame hanger bracket and an axle seat to maintain a substantially constant pitch of the axle and to provide lift mechanisms for such suspensions.
Many devices have been used to lift axle suspensions in the past. FIGS. 1-2 are exemplary illustrations of such devices.
FIG. 1 depicts a conventional twin beam lift axle suspension which includes an air bellows 9 which angularly expands thereby raising trailing arm 21 and axle 7 attached thereto. This raising of the axle lifts the wheels of the system from engagement with the road surface. An appendage 19 is connected to trailing arm 21, and when the air bellows is expanded or contracted the trailing arm 21 is pivotally rotated about a resilient member 23 which defines a pivot point. The air bellows 9 (or airlift spring) is attached at one end 25 to the appendage 19, with the other end 27 being attached to the fixedly mounted frame hanger bracket 29. When the air bellows 9 is inflated, it reacts against the frame bracket 29 and the beam appendage 19 causing the trailing arm 21 to move relative to the bracket 29 about the pivot point defined by the resilient member 23. This expansion of the airlift spring causes the trailing arm and axle 7 to be raised and the wheels of the system to be lifted from road engagement. When the airlift spring is deflated, the axle and wheels attached thereto are lowered into engagement with the road surface.
This design is not without its limitations. During lifting, the air bellows 9 is articulated through an angle due to the geometry of the suspension. This angular expansion of the air bellows is sometimes referred to as an "accordion effect". The angle from parallel typically should not exceed 25.degree. due to restrictions in the air spring construction. Due to this accordion effect, the effectiveness and lifting capabilities of the air bellows are limited and maintenance requirements are increased, as more fully discussed below.
The air bellows 9 in this design is typically located within the frame hanger bracket 29 of the suspension. Due to the size and angular expansion of the air spring, the frame bracket 29 must be of a size larger than is otherwise necessary, thus adding to the cost and weight of the suspension. Another problem with the angular expansion of the air spring in this system is that the air spring undergoes added wear and tear due to the non-symmetric nature of the angular expansion (i.e. the "accordion effect"). In this respect, the upper side of the air spring 9 shown in FIG. 1 realizes higher stresses than the lower side due to stretching of the spring resulting from the accordian effect. Accordingly, because of the accordian effect, increased maintenance is required in more frequently replacing the air springs, which could be avoided if the air springs were to be expanded in a uniform symmetrical manner keeping the end plates 25 and 27 substantially parallel at all times during articulation of the air spring.
Furthermore, as can be seen in FIG. 1, the air spring or bellows 9 has one end 27 fixedly attached to fixed frame bracket 29 resulting in a uni-directional expansion of the bellows 9. Accordingly, during the uni-directional expansion of the bellows 9, the one end 27 remains fixed while the other end 25 moves to the left as shown in FIG. 1, thus driving appendage 19 and arm 21. As a result, substantially 100% of the expansion forces exerted by bellows 9 are directed toward appendage 19 and arm 21 thus requiring arm 21 to be of a size large enough to continually withstand 100% of the expansion forces of bellows 9.
There has also been a development of another type of suspension for special applications that may be referred to as a "parallelogram" lift axle suspension. An example of this type of lift axle suspension is shown in FIG. 2a and 2b. The parallelogram lift axle suspension is used primarily, but not exclusively, in steerable type lift axle suspension systems which are self-steering. The self-steering results from the liftable axle 39 being installed with the proper caster or pitch and the drag of the wheels attached to the axle causing the axle to steer automatically in response to steering of the vehicle.
The reason for the popularity of the parallelogram suspension is its ability to keep the caster (i.e. angular relationship of the axle 39 relative to the ground or the horizontal) or pitch of the axle relatively constant through vertical articulation of the axle. In other words, as the axle 39 and wheels (not shown) attached thereto are lowered or raised to or from engagement with the road surface, the caster angle (or pitch) of the axle 39 remains substantially constant.
The parallelogram lift axle suspension system shown in FIGS. 2a and 2b is typical for the industry and includes an uni-direction expanding air bellows 47, a downwardly extending frame hanger bracket 31, a lower trailing beam 33, an upper control arm 35, and an axle seat 37 connected to the axle 39. Uni-directional expansion of the air bellows 47 causes the beam 33 and arm 35 to pivot upward and lift the axle and wheels attached thereto from road engagement. Castor or pitch remains substantially constant throughout the axles raising and lowering due to the parallelogram design.
The uni-directional expansion results from end plate 51 of bellows 47 being attached to fixed hanger bracket 31. Accordingly, when bellows 47 is expanded [i.e. FIG. 2a], end plate 51 remains fixed while end plate 49 is moved to the left as shown in FIG. 2 thereby driving beam 33 and raising the axle 39 from engagement with the road surface.
The lower trailing beam 33 is pivotally attached to the frame bracket 31 at one end and pivotally attached to the axle seat 37 at the other end. The upper control arm 35 of this conventional lift axle suspension is pivotally attached to both the frame bracket 31 and the axle seat 37 at pivot points 38. The frame bracket 31, lower trailing beam 33, axle seat 37, and upper control arm 35 are all pivotally connected to form the parallelogram that allows the axle 39 to have relatively constant caster or pitch throughout its vertical articulation.
In a fashion similar to that of the system illustrated in FIG. 1, the air bellows 47 of FIGS. 2a-2b undergoes an accordian effect during expansion and contraction (shown in FIG. 2b). Of course, as discussed above, one end 51 of bellows 47 is attached to fixed frame bracket 31 and the other end 49 attached to appendage 43. This results in the uni-directional expansion of bellows 47. The movement of lower arm 33, resulting from bellows 47 being expanded and contracted, necessarily causes upper arm 35 to pivot simultaneously due to the linking of the upper and lower arms via axle seat 37. Because substantially all of the expansion forces of lifting bellows 47 are directed toward the lower arm 33 via elements 41 and 43, the lower arm 33 must be large enough so as to safely transmit all of such expansion forces.
The system of FIGS. 2a-2b is exemplary of several known parallelogram lift systems. Typically, such parallelogram type suspensions provides more lift height capability than a standard twin beam lift. In the design of the parallelogram system of FIGS. 2a and 2b, for example, the appendage 41 is attached to the lower trailing beam or control arm 33 and an intermediate bracket 43 is pivotally attached to the appendage at a pivot point 45. The air spring or air bellows 47 is attached at one end to the intermediate bracket 43 and at the other end to the frame bracket 31. The twin beam lift system (e.g. FIG. 1) lowers and raises its beams in a similar fashion, except it has no counterpart to the intermediate bracket 43 and the appendage 41 in a parallelogram suspension which allow more lift before the angular capabilities of the lift air spring 47 are exceeded. This is because the addition of the intermediate bracket and the appendage allows the bellows 47 to undergo less of an accordian effect than that of FIG. 1 throughout expansion and contraction. Nevertheless, the accordian affect is not entirely eliminated.
A disadvantage of the parallelogram type lift system shown in FIGS. 2a and 2b over that shown in FIG. 1 arises from the necessity to employ added pivot point 45 to join appendage 41 to intermediate bracket 43. Once again, as well, the uni-directional expansion/contraction of bellows 47 must be taken into account in the design. For example, the addition of pivot point 45 adds a potential wear point to the suspension and thus increases maintenance requirements. This disadvantage cannot be eliminated because pivot point 45 is needed.
The resulting uni-directional expansion of bellows 47, of course, gives rise to the need for a substantially large lower control arm 33 and a fairly large expansion bellows 47.
One variation of the parallelogram lift axle suspension shown in FIGS. 2a and 2b utilizes a turn buckle (not shown) adjustment mechanism, including multiple fasteners, in the upper control arm 35 which is used to effectually adjust the length of the upper control arm and therefore adjust the caster angle of the axle 39 through tilting of the axle seat. This allows for the caster (or pitch) of the axle to be changed as needed. For example, a truck heavily loaded in the rearward portion thereof has its rear end closer to the road surface than would an empty truck, because as additional weight is added to the rear section of the trailer, the truck is forced downward at its rear section and closer to the road surface. This changes the preferred caster of the axle and creates a need for caster adjustment.
The disadvantage of the turn buckle adjustment mechanism described above is that the number of fasteners on the upper control arm is increased by about four thereby increasing the number of fasteners which must be torqued and maintained.
The parallelogram lift axle suspension system illustrated in FIGS. 2a and 2b also includes a shock absorber 53 mounted to appendage 57 and to the frame of the vehicle via a shock mount 55. Often, it is desirable to install shock absorbers on the parallelogram suspension 50 so as to dampen vibration of the axle and the control arms. FIGS. 2a and 2b show a typical shock absorber installation where appendage 57 extends from the rear of the axle 39 with the lower end of the shock absorber 53 being connected to the appendage 57 and the upper end of the shock absorber being connected to the upper shock mount 55.
The disadvantages of this type of shock absorber installation are as follows. The upper shock mount must be installed by the installer of the suspension and the presence of the appendage 57 is required. The location of the shock absorber 53 shown in FIG. 2, rearward of the axle 39, is not ideal and causes the shock absorber to wear faster than desirable. Furthermore, the location of the shock absorber 53 shown in FIG. 2 requires substantial space below the vehicle chassis that may not always be available on certain kinds of vehicles.
U.S. Pat. No. 5,018,756 discloses a suspension system in which a uni-directional airlift spring (or air bellows) angularly expands and contracts thereby undergoing an accordion effect. This reduces the effectiveness and lifting capabilities of the air spring as discussed above. This patent does not disclose or suggest how one could eliminate the uni-directional angular expansion and contraction of the bellows.
U.S. Pat. No. 3,861,708 teaches the use of uni-directional expanding and contracting air bellows or air springs to lower and raise an axle in the context of a lift axle suspension system. This patent teaches the advantage of avoiding an accordion effect by locating the center of gravity of the element to be moved as close to the longitudinal axis of the air bellows as possible. However, the uni-directional expanding air springs of this patent are not used in conjunction with a twin beam parallelogram type lift axle suspension.
It is apparent from the above that there exists a need in the art for a better means for lifting the axles in parallelogram type axle suspensions that would reduce costs and weight, provide for a bi-directional expansion and contraction of the bellows, reduce the relative sizes of the lower control arms and air bellows, increase lift capabilities, and reduce maintenance. It is also apparent that there exists a further need in the art to provide a simpler caster angle adjustment mechanism and a more efficient shock absorber installation means that would be more cost effective, durable, and provide for decreased maintenance requirements.